Estimating Avogadros Number Chemistry 141 Lab Handout Stearic

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Estimating Avogadro’s Number Chemistry 141 Lab Handout

Estimating Avogadro’s Number Chemistry 141 Lab Handout

Stearic Acid Monolayer of stearic acid molecules

Stearic Acid Monolayer of stearic acid molecules

Stearic Acid Thickness = 5. 44 x m xm Volume of 1 stearic acid

Stearic Acid Thickness = 5. 44 x m xm Volume of 1 stearic acid molecule = length x width x height = (x m) (5. 44 x m) =5. 44 x 3 m 3

Part 1: Calibration of the pipet. • Wash and dry a 10 m. L

Part 1: Calibration of the pipet. • Wash and dry a 10 m. L beaker. – – – 3 x ammonia wash 3 x deionized water rinse 1 x acetone rinse Dry well with paper towel 3 x hexane rinse (1/2 m. L per rinse) ALL WASTE HEXANE MUST BE DISPOSED OF IN THE WASTE CONTAINERS. DO NOT DUMP HEXANE IN THE SINK. • Fill 10 m. L beaker with hexane. • Transfer hexane drop-wise to a clean dry 10 -m. L graduated cylinder to reach a volume of 1 ml. Record the number of drops. • Repeat until 2 trials are within 10% of each other. • Dispose of excess hexane in the appropriate waste container. • Calculate average volume per drop of hexane.

Sample Data Trial Drops of hexane/m. L Average Drops hexane/m. L Average m. L/drop

Sample Data Trial Drops of hexane/m. L Average Drops hexane/m. L Average m. L/drop hexane 1 18 19 0. 052 m. L 2 20 3 19

Part II: The Monolayer. • Clean a watchglass with soap and water, ammonia, and

Part II: The Monolayer. • Clean a watchglass with soap and water, ammonia, and deionized water. Hold the glass by the edges to avoid putting fingerprints on it. • Place the watch glass on a ring and make sure that the watch glass is parallel to the floor. Fill the watch glass to the brim with deionized water. • Fill clean 10 m. L beaker with stearic acid/hexane solution from under the hood. • Add stearic acid/hexane dropwise to the watchglass. Count the drops. – The solution should spread out rapidly across the surface of the water and disappear within a few seconds. If the watch glass is not properly cleaned then an oily residue may appear after only a few drops of solution. In this case it will be necessary to clean the watch glass again. If the first few drops disappear rapidly, continue adding the solution drop-wise, counting the drops. After approximately 20 drops, as the monolayer nears completion, the drop of solution forms a circular pattern rather than flowering out. The circular film of solution contracts as it evaporates and disappears in a relatively short time. This pattern will be observed for a few drops until finally, one drop strikes the surface and remains as a lens or globule that requires a prolonged period of time to disappear. Record the number of drops. At this point, the surface of the water is covered with a monolayer of stearic acid and one more drop placed at a different point on the water surface forms a second “lens”.

Sample Calculations Trial 1 Drops hexane/m. L 19 Number of drops of solution used

Sample Calculations Trial 1 Drops hexane/m. L 19 Number of drops of solution used for monolayer 42 Stearic Acid concentration 0. 10 g/m. L Mass of Stearic Acid in Monolayer 2. 2 x 10 -4 g SA

Sample Calculations Trial 1 Mass of Stearic Acid in Monolayer 2. 2 x 10

Sample Calculations Trial 1 Mass of Stearic Acid in Monolayer 2. 2 x 10 -4 g SA Density of Stearic Acid 0. 870 g/m. L Volume of Stearic Acid in Monolayer 2. 5 x 10 -4 m. L SA

Sample Calculations Trial 1 Diameter of water surface 15. 3 cm Area of water

Sample Calculations Trial 1 Diameter of water surface 15. 3 cm Area of water surface 184 cm 2

Sample Calculations Trial 1 Volume of Stearic Acid in Monolayer 2. 5 x 10

Sample Calculations Trial 1 Volume of Stearic Acid in Monolayer 2. 5 x 10 -4 m. L SA Area of water surface 184 cm 2 Thickness of monolayer 1. 4 x 10 -6 cm

Sample Calculations Trial 1 Thickness of monolayer 1. 4 x 10 -6 cm Height

Sample Calculations Trial 1 Thickness of monolayer 1. 4 x 10 -6 cm Height of SA molecule 1. 4 x 10 -6 cm Width of SA molecule 2. 5 x 10 -7 cm Volume SA molecule 8. 7 x 10 -20 cm 3

Sample Calculations Trial 1 Volume of Stearic Acid in Monolayer 2. 5 x 10

Sample Calculations Trial 1 Volume of Stearic Acid in Monolayer 2. 5 x 10 -4 m. L SA Volume SA molecule 8. 7 x 10 -20 m. L Number SA molecules in monolayer 2. 9 x 1015

Sample Calculations Trial 1 Mass of SA in Monolayer 2. 2 x 10 -4

Sample Calculations Trial 1 Mass of SA in Monolayer 2. 2 x 10 -4 g SA Molar Mass SA 284 g/mol Moles of Stearic Acid in Monolayer 7. 7 x 10 -7 mol SA

Sample Calculations Trial 1 Number SA molecules in monolayer 2. 9 x 1015 molecules

Sample Calculations Trial 1 Number SA molecules in monolayer 2. 9 x 1015 molecules SA Moles of Stearic Acid in Monolayer 7. 7 x 10 -7 mol SA Experimental value of Avogadro’s number 3. 8 x 1021 molec/mol Not bad for random numbers!!